Reports about the draft of Trump’s letter first came to light in a New York Times report on September 1, and claimed the content of the letter prove that the president fired Comey to quell the Russia investigation.

In his interview, Castro said that the House Intelligence Committee on which he sits has not yet reviewed the letter currently in the possession of special counsel Robert Mueller, but the reports about it could trigger impeachment proceedings if proven true.

“I said early on that if President Trump fired Comey [to kill the Russia probe], then that is grounds to start impeachment proceedings,” Castro said. “If he fires Mueller for a similar reason, to get rid of the Russia investigation, or because he thinks that it will accomplish that, those are grounds to start impeachment proceedings.”

He went on to agree with Melber’s assertion that the reported contents of the letter could constitute a “red line” that the president may already have crossed.